The invention relates to a method for balancing clutch wear in a dual clutch transmission, wherein clutch wear can be balanced by determining which clutch is most frequently used for transferring power and selecting a gear actuated by the other clutch during launch of the vehicle.
In recent years the use of dual clutch transmissions (DCT) has increased in vehicles, also for commercial vehicles. A typical dual clutch transmission configuration includes a pair of mutually exclusively operating input clutches which selectively drive a pair of input shafts.
The input shafts may be disposed on opposite sides of an output shaft or may be disposed concentrically between spaced-apart output shafts. One of each of a plurality of pairs of constantly meshing gears which provide the various forward and reverse gear ratios is freely rotatably disposed on one of the shafts and the other of each pair of gears is coupled to one of the other shafts. A plurality of tooth clutches can selectively connect the freely rotatable gears to the associated shaft.
Dual clutch transmissions provide operating characteristics which can combine those of both conventional automatic planetary transmissions and manual transmissions. An advantageous characteristic is the power-shift function, which allows gear shifts with uninterrupted torque transfer to suitable ground engaging members, such as driving wheels or tracks. Dual clutch transmissions also exhibit good fuel economy due to their good gear mesh efficiency, ratio selection flexibility and reduced clutch wear.
A problem with dual clutch transmissions is that one of the clutches may be subjected to more wear than the other clutch. For instance, during long distance driving, power shifting between the top gears can cause increased wear for the clutch used for shifting into the top gear. Uneven wear of the clutches will eventually lead to repair or replacement of the worn clutch, before the other clutch is sufficiently worn to warrant repair. Disassembly of the transmission to service the clutches is time consuming and expensive, which can lead to unnecessary replacement of the less worn clutch merely to avoid an additional servicing at a later date when the latter clutch is worn out.
This problem occurs in all such dual clutch transmissions, dry and wet, but is particularly noticeable for dry clinches.
It is desirable to provide an improved method tier controlling the wear of a dual clutch transmission that solves or minimises the above problems.
In the subsequent text, the term “launch” is intended to describe a situation where a vehicle is being started from standstill. The launch ends when the transmission is changed into a subsequent gear higher than the gear used during launch. A gear change can involve selecting the next gear in a sequence of gears (1st gear to 2nd gear, etc.) or by skipping one or more gears in the sequence (1st gear to 3rd gear, etc.). The gear selection can, for instance, depend on vehicle weight and whether the vehicle is launched uphill or downhill.
According to a preferred embodiment, an aspect of the invention relates to a method for balancing clutch wear in a dual clutch transmission. The transmission comprises a first transmission mechanism arranged to transmit mechanical driving force from an output shaft of a prime mover, such as an internal combustion engine, to a first input shaft and to put any one of a first set of gears in an engaged state to connect the first input shaft to ground engaging members, such as driving wheels or tracks. In the subsequent text, the invention will be described for use with ground engaging members in the form of driving wheels. The transmission further comprises a second transmission mechanism arranged to transmit mechanical driving three from the output shaft of the prime mover to a second input shaft and to put any one of a second set of gears in an engaged state to connect the second input shaft to the driving wheels. A controllable first clutch unit is arranged to bring the output shaft of the prime mover and the first input shaft into engagement with each other. Similarly, a controllable second clutch unit is arranged to bring the output shaft of the prime mover and the second input shaft into engagement with each other. An electronic control unit is arranged to control gear selection and clutch unit actuation.
The method is carried out by performing the steps of                determining which of the first and second clutch unit is subjected to the most accumulated wear; and        selecting a gear for launch actuated by the other clutch unit than the above determined clutch unit during launch of the vehicle.        
According, to the invention, the method involves selecting a gear higher than the lowest gear during launch of the vehicle, if possible. Under circumstances where this selection is not possible for reasons of drivability, the electronic control unit is arranged to override this gear selection and allow selection of a gear required for launch of the vehicle. This situation can occur when, for instance, the vehicle is launched uphill and cannot be started in a gear higher than the lowest gear. Another example is when the vehicle is to be driven at very low speed, e.g., at low-speed maneuvers for docking a trailer.
The invention is preferably, but not necessarily, related to a method for balancing clutch wear in a dual clutch transmission (DCT). Clutch wear mainly occurs during launch from standstill and during long distance driving, when shifting between the top gears. In particular, power shifting between the top (highest and second highest) gears can cause increased wear for the clutch used for shifting into the top gear. Power shifting allows gear shifts with uninterrupted torque transfer to the driving wheels. In this case clutch wear can be balanced by using the clutch not transferring power in the top gear for launching the vehicle to a greater extent. Preferably, the clutch not transferring power in the top gear is selected for launching, the vehicle during 50-100% of the launches. For example, in a 12-speed DCT gearbox where a first clutch controls odd gears (gears 1, 3, 5, etc.) and a second clutch controls even gears (gears 2, 4, 6, etc.), power shifting at cruising speeds will cause more wear on the second clutch, In order to balance clutch wear, at least half and preferably all launches are performed using an uneven gear controlled by the first clutch. The method is used when starting the vehicle in a gear higher than the lowest (1st) gear. When it is desired to launch the vehicle in a gear higher than the lowest gear. gear. the vehicle will be launched in one of the gears 3, 5, etc. By avoiding gears 2, 4, etc. it is possible to balance clutch wear.
Clutch wear can also occur in gears other than the top gear. Examples of such cases are vehicles frequently operated in cities, on construction sites, on minor roads, etc. where the cruising speed is rarely reached and the vehicle is predominantly used in intermediate gears between the top gear and gears used for launch. In order to determine which clutch is subjected to the most accumulated wear, further method steps are carried out involving:                detecting at least one wear related transmission parameter indicative of clutch wear during operation of the vehicle;        determining which of the first and second clutch unit is subjected to the most wear using the detected parameter; and        selecting a gear actuated by the other clutch unit than the above determined clutch unit during launch of the vehicle.        
Examples of wear related transmission parameters are the number of gear shifts performed by each clutch, or the amount of accumulated slip energy transferred by each clutch. Depending on factors such as local road conditions, speed restrictions, the driving style of the driver and the typical use of the vehicle, one of the clutches will experience more wear. As opposed to the first case described above, the clutch subjected to most wear need not be the one used for the top gear. In fact, the clutch in question can be either of the first and second clutches, and the clutch subjected to the most wear can vary over time, for instance if the vehicle is being used for different purposes.
Once it has been determined which clutch is subjected to the most wear, a gear actuate by the other clutch is selected during at least 50% of all launches of the vehicle. As an alternative, the other clutch is used for at least 50% of the accumulated clutch slip energy dissipated at launches. As a further alternative, the other clutch is used for at least 50% of the accumulated clutch slip energy dissipated during gear changes.
The method according to the invention as indicated above is preferably applicable on vehicle transmissions having eight or more gears. The method is preferably, but not necessarily, applicable on dual clutch transmissions comprising dry clutches. The invention is primarily intended for commercial vehicles, such as trucks, buses and work vehicles. In such vehicles, it is common practice to launch not exclusively in the lowest gear under certain conditions, such as unladen vehicle and/or downhill road inclination. By doing so, the acceleration of the vehicle is improved, less fuel is consumed and the wear on shift control elements is reduced. This is especially the case if there are a large number of gears in the transmission, e.g., eight gears or more.
The present invention also relates to a computer program, computer program product and a storage medium for a computer all to be used with a computer for executing the method as described in any one of the above examples The computer program comprises program code means for performing all the steps of the above method when said program is run on a computer. The computer program product comprises program code means stored on a computer readable medium for performing all steps of anyone of the above method when said program product is run on a computer. The storage medium, such as a computer memory or a non-volatile data storage medium, is intended for use in a computing environment, the memory comprising a computer readable program code to perform the method.